Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
The disclosure relates to a fixing device that employs a belt fixing method where a paper sheet carrying an unfixed toner image is inserted into a fixing nip portion, which is formed by a heated fixing belt and a pressure member, and the unfixed toner is heated and melted for fixation on the paper sheet. The disclosure also relates to an image forming apparatus including the fixing device employing an electrophotographic method.
In the conventional image forming apparatus that employs the electrophotographic method, the following belt fixing method has been developed. Instead of a heating roller, an endless fixing belt that absorbs radiant heat from a heat source and generates heat is employed as a heating member for heating the paper sheet. The paper sheet carrying an unfixed toner image is inserted into a fixing nip portion formed by the heated fixing belt and a pressure member, which is brought into pressure contact with the fixing belt, thus a toner is fixed on the paper sheet.
In this belt fixing method, at least one of a fixing roller pair forming the fixing nip portion is used as a heating roller. Inserting the paper sheet carrying the unfixed toner image into the fixing nip portion can decrease thermal capacity and shorten a warm-up period, thus reducing power consumption, compared with a heat roller fixing method fixing a toner on a paper sheet.
The following methods for driving the fixing belt are known, for example. Flange-shaped end cap members are secured on both ends of the endless fixing belt in a rotation shaft direction. The fixing belt is driven via a gear formed at the end cap members. Alternatively, the fixing belt is driven with a suspension roller disposed downstream of a nip portion inside of the endless fixing belt.
However, with the above-described method of directly driving the fixing belt, pressing members, such as the end cap member and the suspension roller disposed inside of the fixing belt, may need to be rotated. Accordingly, it was difficult to freely configure a shape and a width of the nip portion.
As a method for expanding a nip width, for example, a method of using a pressure roller with large diameter, a method of increasing rubber thickness or reducing rubber hardness at a surface of the pressure roller, or a method of enhancing pressing force by the pressure roller are generally known. However, the pressure roller with large diameter may result in large-size fixing device and a cost increase, whereas an increase in rubber thickness may result in extension of the warm-up period. Reduction in rubber hardness increases a change in outer diameter due to temperature, causing reduction in conveyability, also degrading durability. Further, elevation in the pressing force by the pressure roller leads to reduction in conveyability due to excessive amount of deflection of the roller surface and a cost increase due to reinforcement of a fixing frame.
Therefore, the following sliding-belt fixing method has been devised. A supporting member is disposed inside of the fixing belt. A pressure roller is brought into pressure contact with the supporting member from outside of the fixing belt. At the same time, a friction force between the pressure roller and the outer surface of the fixing belt slides the supporting member and the inner surface of the fixing belt, thus rotating the fixing belt.
The following fixing device has been disclosed, for example. The fixing device includes a fixing belt, a radiant heat source (halogen heater) inside of the belt, a supporting member with a sliding surface, and a pressure roller. Rotatably driving the pressure roller to slide a fixing belt and the supporting member at a nip portion formed by the fixing belt and the pressure roller, thus rotating the fixing belt.
With the above-described sliding-belt fixing method, to rotate the fixing belt smoothly, slidability between the fixing belt and the supporting member may need to be ensured. Accordingly, another fixing belt has been disclosed. A sliding layer, which forms a sliding surface on a side sliding along the supporting member of the fixing belt (inner circumferential surface), is disposed. This ensures the improved wear resistance and slidability of the belt inner circumferential surface.